SYNOPSIS

DESCRIPTION

The pthread_mutexattr_destroy() function shall destroy a mutex attributes object; the object becomes, in effect,
uninitialized. An implementation may cause pthread_mutexattr_destroy() to set the object referenced by attr to an
invalid value. A destroyed attr attributes object can be reinitialized using pthread_mutexattr_init(); the results of
otherwise referencing the object after it has been destroyed are undefined.

The pthread_mutexattr_init() function shall initialize a mutex attributes object attr with the default value for
all of the attributes defined by the implementation.

Results are undefined if pthread_mutexattr_init() is called specifying an already initialized attr attributes
object.

After a mutex attributes object has been used to initialize one or more mutexes, any function affecting the attributes object
(including destruction) shall not affect any previously initialized mutexes.

RETURN VALUE

Upon successful completion, pthread_mutexattr_destroy() and pthread_mutexattr_init() shall return zero; otherwise,
an error number shall be returned to indicate the error.

ERRORS

The pthread_mutexattr_destroy() function may fail if:

[EINVAL]

The value specified by attr is invalid.

The pthread_mutexattr_init() function shall fail if:

[ENOMEM]

Insufficient memory exists to initialize the mutex attributes object.

These functions shall not return an error code of [EINTR].

The following sections are informative.

EXAMPLES

None.

APPLICATION USAGE

None.

RATIONALE

See pthread_attr_init() for a general explanation of attributes. Attributes objects
allow implementations to experiment with useful extensions and permit extension of this volume of IEEE Std 1003.1-2001
without changing the existing functions. Thus, they provide for future extensibility of this volume of
IEEE Std 1003.1-2001 and reduce the temptation to standardize prematurely on semantics that are not yet widely
implemented or understood.

Examples of possible additional mutex attributes that have been discussed are spin_only, limited_spin,
no_spin, recursive, and metered. (To explain what the latter attributes might mean: recursive mutexes would
allow for multiple re-locking by the current owner; metered mutexes would transparently keep records of queue length, wait time,
and so on.) Since there is not yet wide agreement on the usefulness of these resulting from shared implementation and usage
experience, they are not yet specified in this volume of IEEE Std 1003.1-2001. Mutex attributes objects, however, make it
possible to test out these concepts for possible standardization at a later time.

Mutex Attributes and Performance

Care has been taken to ensure that the default values of the mutex attributes have been defined such that mutexes initialized
with the defaults have simple enough semantics so that the locking and unlocking can be done with the equivalent of a test-and-set
instruction (plus possibly a few other basic instructions).

There is at least one implementation method that can be used to reduce the cost of testing at lock-time if a mutex has
non-default attributes. One such method that an implementation can employ (and this can be made fully transparent to fully
conforming POSIX applications) is to secretly pre-lock any mutexes that are initialized to non-default attributes. Any later
attempt to lock such a mutex causes the implementation to branch to the "slow path" as if the mutex were unavailable; then, on
the slow path, the implementation can do the "real work" to lock a non-default mutex. The underlying unlock operation is more
complicated since the implementation never really wants to release the pre-lock on this kind of mutex. This illustrates that,
depending on the hardware, there may be certain optimizations that can be used so that whatever mutex attributes are considered
"most frequently used" can be processed most efficiently.

Process Shared Memory and Synchronization

The existence of memory mapping functions in this volume of IEEE Std 1003.1-2001 leads to the possibility that an
application may allocate the synchronization objects from this section in memory that is accessed by multiple processes (and
therefore, by threads of multiple processes).

In order to permit such usage, while at the same time keeping the usual case (that is, usage within a single process) efficient,
a process-shared option has been defined.

If an implementation supports the _POSIX_THREAD_PROCESS_SHARED option, then the process-shared attribute can be used to
indicate that mutexes or condition variables may be accessed by threads of multiple processes.

The default setting of PTHREAD_PROCESS_PRIVATE has been chosen for the process-shared attribute so that the most
efficient forms of these synchronization objects are created by default.

Synchronization variables that are initialized with the PTHREAD_PROCESS_PRIVATE process-shared attribute may only be
operated on by threads in the process that initialized them. Synchronization variables that are initialized with the
PTHREAD_PROCESS_SHARED process-shared attribute may be operated on by any thread in any process that has access to it. In
particular, these processes may exist beyond the lifetime of the initializing process. For example, the following code implements a
simple counting semaphore in a mapped file that may be used by many processes.

The following code is for three separate processes that create, post, and wait on a semaphore in the file /tmp/semaphore.
Once the file is created, the post and wait programs increment and decrement the counting semaphore (waiting and waking as
required) even though they did not initialize the semaphore.